This invention relates to methods and apparatus for anamorphically shaping and deflecting beams of electromagnetic radiation, for example, a radiation beam produced by a diode laser. More particularly, it relates to a method and apparatus for achromatically deflecting a beam by precisely 90.degree. while anamorphically altering its intensity profile.
The advent of the diode laser has greatly expanded the need to handle laser beams of elliptical cross section. Because these lasers have very asymmetric active regions, they may exhibit beam divergence ratios in the 1:2-1:4 range. When the beam is collimated by a collector lens, it retains its ellipticity. The beam ellipticity, in turn, affects (usually reduces) the efficiency with which the beam can be focused through a circular objective lens.
Heretofore, it has been common to use a Littrow-type prism (i.e., a three sided right angle prism) for anamorphically expanding an elliptical beam to make it more circular. The output of a diode laser is predominantly polarized in the TE state, and the collimated beam is polarized parallel to its minor axis. If, as shown in FIG. 1, the beam B is incident at the Brewster angle .THETA..sub.B, on the hypotenuse face of a Littrow prism in which one of the acute angles is cut at the Brewster angle, it will be anamorphically expanded by a factor n in a direction of the minor axis, where n is the refractive index of the prism. While this is a good approach for reducing the ellipticity of a diode laser beam, it is disadvantageous from the standpoint that the beam is deflected by an inconvenient angle .alpha., where .alpha.=2.THETA..sub.B -90.degree.. For ordinary glass prisms, .alpha. is between 20.degree. and 30.degree.. For many applications, it is desirable that the expanded beam be either parallel to, if not co linear with, the incoming beam or, alternatively, be deflected during the expansion by some convenient angle, for example, 90.degree.. Moreover, it is highly desirable that the deflection angle be independent of changes in wavelength of the incoming beam. In the case of diode lasers, for example, shifts in wavelength of 20-30 nm are common in responses to ambient temperature changes, variations in the applied voltage, and optical feedback noise.
In U.S. Pat. No. 4,759,616 issued in the name of A. B. Marchant, there is disclosed a method for anamorphically shaping a beam of radiation and for deflecting such beam by precisely 90.degree.. This method also makes use of a Littrow type prism (or a truncated version of such prism) but, unlike the optical arrangement described above with reference to FIG. 1, the prism is arranged in the beam path so that the beam enters the prism face opposite the .THETA..sub.B angle at (or near) the Brewster angle. Referring to FIG. 2, upon entering the prism, the refracted beam is reflected by that prism face opposite the (90.degree.-.THETA..sub.B) angle and emerges from the prism through the hypotenuse face at 90.degree.. While this beam-shaping and deflecting technique has the advantage of deflecting the beam through a convenient angle, the prism employed can be difficult to optically align with the laser source and other optical elements which operate on the beam as it enters and exits the prism. Moreover, the prism is not achromatic and, hence, the deflecting angle is still wavelength-dependent.